The present invention is directed to the field of sails and methods for their manufacture.
Sails can be flat, two-dimensional sails or three-dimensional sails. Most typically, three-dimensional sails are made by broadseaming a number of panels. The panels, each being a finished sector of sailcloth, are cut along a curve and assembled to other panels to create the three-dimensional aspect for the sail. Traditionally sails have been made out of panels of sailcloth seamed together. Seams are narrow overlaps between panels; they can be stitched, bonded or both. The widths of the overlaps vary accordingly with the design strength of the sail. Typically wider seams are used on more highly loaded sails. The seams are generally aligned with the warp axis of the sailcloth. The seams generally cross the load direction when making cross cut-sails and are generally parallel to the load direction when making radial and tri-radial sails. The panels typically have a quadrilateral or triangular shape with a maximum width being limited traditionally by the width of the roll of finished sailcloth from which they are being cut. Typically the widths of the sailcloth rolls range between about 91.5 and 137 centimeters (36 and 58 inches).
Sailcloth manufacturers have developed low stretch rolls of sailcloth whether woven, non-woven or laminated to help control sail shape. In some woven materials made by Dirnension-Polyant of Germany, larger warp yarns or fill yarns or a combination of both might be combined with finer weave yarns to increase fabric strength.
Sailmakers have tried to take advantage of seam width to enhance the stability of the sail. For instance, U.S. Pat. No. 94,400, issued in 1869 to Crandall, shows the use of radiating seams out of the clews to bear strain and improve the set of the sail. During the 1970's while building cross-cut woven sails, Hood sailmakers typically used ½ width panels to increase the number of seams and therefore the percentage of overlap throughout the body of the sail. Later and since the 1980's sailmakers building tirradial sails aligned the seams tangent with the loads to increase stability of the sail. One of the benefit was to be able to reduce somewhat the weight of the sailfabric used compared to cross-cut constructions.
Sailmakers have many restraints and conditions placed on them. In addition to building products which will resist deterioration from weather and chafe abuses, a goal of modern sailmaking is to create a lightweight, flexible, three-dimensional air foil that will maintain its desired aerodynamic shape through a chosen wind range. A key factor in achieving this goal is stretch control of the airfoil. Stretch is to be avoided for two main reasons. First, it distorts the sail shape as the wind increases, making the sail deeper and moving the draft aft. This creates undesired drag as well as excessive heeling of the boat. Second, sail stretch wastes precious wind energy that should be transferred to the sailcraft through its rigging.
Over the years, sailmakers have attempted to control stretch and the resulting undesired distortion of the sail in several additional ways.
One way sailmakers attempted to control sail stretch is by using low-stretch high modulus yarns in the making of the sailcloth. The specific tensile modulus in gr/denier is about 30 for cotton yarns (used in the 1940's), about 100 for Dacron® polyester yarns from DuPont(used in the 1950's to 1970's), about 900 for Kevlar ® para-aramid yarns from DuPont (used in 1980's) and about 3000 for carbon yarns (used in 1990's).
Another way sailmakers have attempted to control sail stretch has involved better yarn alignment based on better understanding of stress distribution in the finished sail. Lighter and yet lower-stretch sails have been made by optimizing sailcloth weight and strength and working on yarn alignment to match more accurately the encountered stress intensities and their directions. The efforts have included both fill-oriented and warp-oriented sailcloths and individual yarns sandwiched between two films.
An approach to control sail-stretch has been to build a more traditional sail out of conventional woven fill-oriented sailcloth panels and to reinforce it externally by applying flat tapes on top of the panels following the anticipated load lines. See U.S. Pat. Nos. 4,593,639 and 5,172,647. While this approach is relatively inexpensive, it has its own drawbacks. The reinforcing tapes can shrink faster than the sailcloth between the tapes resulting in severe shape irregularities. The unsupported sailcloth between the tapes often bulges, affecting the design of the airfoil. Also, when the normally straight tapes are applied along curved load lines, the radially inside yarns are placed in compression while the radially outside yarns are placed in tension so that the radially outside yarns support most of the load thus reducing the efficiency of the reinforcement tapes.
A further approach has been to manufacture narrow cross-cut panels of sailcloth having individual laid-up yarns following the load lines. The individual yarns are sandwiched between two films and are continuous within each panel. See U.S. Pat. No. 4,708,080 to Conrad. Because the individual radiating yarns are continuous within each panel, there is a fixed relationship between yarn trajectories and the yarn densities achieved. This makes it difficult to optimize yarn densities within each panel. Due to the limited width of the panels, the problem of having a large number of horizontal seams is inherent to this cross-cut approach. The narrow cross-cut panels of sailcloth made from individual spaced-apart radiating yarns are difficult to seam successfully; the stitching does not hold on the individual yarns. Even when the seams are secured together by adhesive to minimize the stitching, the proximity of horizontal seams to the highly loaded corners can be a source of seam, and thus sail, failure.
A still further approach has been to manufacture simultaneously the sailcloth and the sail in one piece (membrane) on a convex mold using uninterrupted load-bearing yarns laminated between two films, the yarns following the anticipated load lines. See U.S. Pat. No. 5,097,784 to Baudet. While providing very light and low-stretch sails, this method has its own technical and economic drawbacks. The uninterrupted nature of every yarn makes it difficult to optimize yarn densities, especially at the sail corners. Also, the specialized nature of the equipment needed for each individual sail makes this a somewhat capital-intensive and thus expensive way to manufacture sails.
Another way sail makers have controlled stretch and maintained proper sail shape has been to reduce the crimp or geometrical stretch of the yarn used in the sailcloths. Crimp is usually considered to be due to a serpentine path taken by a yarn in the sailcloth. In a weave, for instance, the fill and warp yarns are going up and down around each other. This prevents them from being straight and thus from initially fully resisting stretching. When the woven sailcloth is loaded, the yarns tend to straighten before they can begin resist stretching based on their tensile strength and resistance to elongation. Crimp therefore delays and reduces the stretch resistance of the yarns at the time of the loading of the sailcloth.
In an effort to eliminate the problems of this “weave-crimp”, much work has been done to depart from using woven sailcloths. In most cases, woven sailcloths have been replaced by composite sailcloths, typically made up from individual laid-up (non-woven) load-bearing yarns sandwiched between two films of Mylar® polyester film from DuPont or some other suitable film. There are a number of patents in this area, such as Sparkman EP 0 224 729, Linville U.S. Pat. No. 4,679,519, Conrad U.S. Pat. No. 4,708,080, Linville U.S. Pat. No. 4,945,848, Baudet U.S. Pat. No. 5,097,784, Meldner U.S. Pat. No. 5,333,568, and Linville U.S. Pat. No. 5,403,641.
See U.S. Pat. Nos. 6,265,047 and 6,302,044.